Disulfuric acid (alternative spelling disulphuric acid) or pyrosulfuric acid (alternative spelling pyrosulphuric acid), also named oleum, is a

sulfur oxoacid Sulfur oxoacids are chemical compounds that contain sulfur, oxygen, and hydrogen. The best known and most important industrially used is sulfuric acid. Sulfur has several oxoacids; however, some of these are known only from their salts (these are ...

. It is a major constituent of fuming sulfuric acid,

oleum Oleum (Latin ''oleum'', meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Oleu ...

, and this is how most chemists encounter it. As confirmed by X-ray crystallography, the molecule consists of a pair of SO₂(OH) groups joined by an oxide.

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It is also a minor constituent of liquid anhydrous sulfuric acid due to the equilibria: :H2SO4<=>H2O + SO3 :SO3 + H2SO4<=> H2S2O7 :2H2SO4 <=> H2O + H2S2O7 ''Global The acid is prepared by reacting excess

sulfur trioxide Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...

(SO₃) with sulfuric acid:
H2SO4 + SO3 -> H2S2O7 Disulfuric acid can be seen as the sulfuric acid analog of an

acid anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid. In organic chemistry, organic acid anhydrides contain the functional group R(CO)O(CO)R'. Organic acid anhydrides often form when one equivalent ...

. The mutual electron-withdrawing effects of each sulfuric acid unit on its neighbour causes a marked increase in acidity. Disulfuric acid is strong enough to

protonate In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry aci ...

"normal" sulfuric acid in the (anhydrous) sulfuric acid solvent system. There are salts of disulfuric acid, commonly called

pyrosulfate In chemistry, disulfate or pyrosulfate is the anion with the molecular formula . Disulfate is the IUPAC name. It has a dichromate-like structure and can be visualised as two corner-sharing SO4 tetrahedra, with a bridging oxygen atom. In this a ...

s, e.g.

potassium pyrosulfate Potassium pyrosulfate, or potassium disulfate, is an inorganic compound with the chemical formula K2S2O7. Production Potassium pyrosulfate is obtained by the thermal decomposition of other salts, most directly from potassium bisulfate: : 2 KHSO4 ...

. There are other related acids with the general formula H₂O·(SO₃)_''x'' though none can be isolated.

References

{{Hydrogen compounds Hydrogen compounds Pyrosulfates Sulfur oxoacids Acid anhydrides

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See also

References